The majority of vehicle transport refrigeration systems in use today are powered by an internal combustion engine running on diesel fuel, either directly with an auxiliary generator mounted on the refrigerated trailer, or indirectly by taking power from the tractor engine unit mechanically or electrically via an alternator. Cooling is then attained through using that power to drive a standard closed loop refrigeration system.
Typically, both the power take-off and refrigeration unit are over specified for the level of cooling typically required to maintain the compartment temperature in transit. This is for a number of reasons:                i) The refrigeration unit must be capable of cooling down the container after the doors have been opened;        ii) The insulation performance of such cold compartments degrades by 3-5% per year, increasing the cooling power required through the lifecycle; and        iii) APT mandate that the refrigeration unit must be able to extract heat at 1.35 to 1.75 times the heat transfer through the container wall at a 30° C. ambient temperature.        
The result of this is that the refrigeration units on mobile vehicles spend much of their operational lives running at an inefficient point. The consequence of this is that coefficients of performance of mobile refrigeration units are typically quite low compared to other cooling equipment (e.g. approximately 0.5 for frozen compartments at −20° C. to 1.5-1.75 for compartments refrigerated to 3° C.).
Currently, it is estimated that approximately 0.05% of total greenhouse gas emissions in the UK come from the refrigeration equipment used for food transportation. This is a small proportion but represents a significant quantity. Consequently, there is a need to reduce emissions from refrigerated transport units. The inefficient use of hydrocarbon fuels for these refrigeration units is also disadvantageous and so a method of reducing their consumption in this application is required.
A number of alternative cooling methods have been proposed. These include energy storage via fuel cells or battery electric for which the cost, infrastructure and charge time drawbacks are known to be undesirable. Eutectic beams employing phase change materials have been used to store cold, but these impose a significant weight penalty. Adsorption and absorption methods utilizing tractor power unit waste heat are known but tend to be bulky and rely on high quality heat from the tractor power unit which may not be available at idle. Air cycle refrigeration systems using air from the cold compartment as the working fluid remove the need for refrigerants, but still require a power source.
Various cryogenic systems have been described whereby a cryogenic fluid such as liquid nitrogen is stored in an insulated vessel and used as a source of cold. These can be grouped generally as systems which use the cryogen directly by spraying it into the cold compartment, as described in WO 2011/126581 and U.S. Pat. No. 3,699,694, systems that use the cryogen indirectly via a heat exchanger, as described in WO 2010/128233 and WO 01/53764, or a mixture of both. It is also known to use a cryogen with an independently powered refrigeration system to reduce the quantity of cryogen that must be carried. In EP 0599612, the cryogen exchanges heat directly with the refrigerant in a slurry tank. The potential for using a heated or vent vapour from indirect heat exchange to drive an air displacement fan has been considered in WO 2007/116382 and EP 0599626.
However, direct use of the cryogen can pose an asphyxiation hazard with many choices of cryogenic fluid. Moreover, existing cooling systems using cryogenic substances are inefficient. Therefore, there exists a need for a commercially-viable, efficient, safe and sustainable cooling system employing the beneficial properties of cryogenic substances.
It is an aim of the invention to address the above problems.